Influence of Variety and Maturity Level on Natural Convective Heat Drying of Four Onion Varieties Grown in Senegal

The considerable post-harvest losses (5% to 40% depending on storage time) due to the high water content level of the onion varieties in Senegal are the main cause of the problem of access to local onion all year round. Therefore, drying is one of the techniques that can be used to solve the problem of onion perishability. This study deals with the characterization of naturally convective kinetics drying of four onion varieties in relation to their maturity level. The experiment was carried out using the gravimetric method. The Welch and Turkey statistical tests display a significant difference between the effective diffusivity coefficients depending on the maturity level within each variety and across the four varieties. The effective diffusivity coefficients of the Galmi Violet, Safari, Gandiol F1 and Orient F1varieties range from 2.18 × 10−11 ± 2.69 × 10−12 to 1.32 × 10−10 ± 1.17 × 10−11 m2 ·s −1 at a maturity level less than 80%. When the maturity level is greater than 85%, the effective diffusivity coefficients range from 1.30 × 10−11 ± 1.24 × 10−12 to 8.05 × 10−11 ± 8.94 × 10−13 m2 ·s −1 . As far as the activation energy is concerned, the study only reveals a significant difference between the varieties whatever the maturity level is. The Galmi Violet variety stands out with an average activation energy of 66.71 ± 0.12 KJ·mol−1 K−1 for the maturity level below 80% and 58.74 ± 0.11 KJ·mol−1 for the maturity level above 85%. For the three remaining varieties, the average activation energy ranges from 58.15 ± 0.19 to 59.12 ± 0.13 KJ·mol−1 for a maturity level less than 80% whereas the rates go from 47.63 ± 0.28 to 49.96 ± 0.77 KJ·mol−1 when the maturity level is greater than 85%. In summary, the higher the maturity level is, the lower the effective diffusivity coefficients will be. The same tendency was observed with the activation energy. The Galmi Violet variety represents the limitative one in case of the drying of the four varieties mix together. (Authors)